Laser diodes are commonly used to provide optical signals to optical fibers for transmission thereon. Typically, the laser diode will be biased at some selected bias current level, and the diode will then be intensity-modulated about that bias point at a modulation level necessary to achieve a desired light output level. Unfortunately, laser characteristics change in two important ways when operated over a wide temperature range. First, the lasing threshold tends to increase with increasing temperature. This implies that, to maintain a constant average optical output power with an increase in temperature, average drive current, often called bias current, must be increased. Second, the efficiency of the laser current-to-optical power conversion (known as slope efficiency) decreases with increasing temperature.
One implication of the second effect, i.e., the slope efficiency decreasing with increasing temperature, is the same as the first: to maintain at a constant average optical output power with increasing temperature, bias current must be increased. Another implication of the second effect is that, to maintain a constant signal, or modulation, optical output power with increasing temperature, modulation current must be increased.
In order to obtain reliable and repeatable results in many fiber optic transmission applications, both average and signal power out of the laser must be held relatively constant. Many times this problem is skirted through the use of thermo-electric cooling to maintain the laser at a relatively constant temperature. This solution is generally costly, power consumptive, and usually unacceptable for high-volume, low-cost applications. Another possible solution has been to simply monitor the laser temperature and adjust the bias and signal current levels according to expected performance curves. However, for low-cost lasers, the change in characteristics with temperature is usually not accurately predictable from device to device. This mandates that either each laser be individually characterized over temperature, or that a feedback loop be established to control the laser in operation. Individual characterization, besides being expensive, has the additional disadvantage of not accounting for any changes in laser characteristics that may occur as the laser ages.
A feedback loop can be established through the laser's own back facet monitor photodiode, or through the whole link and the receiver at the opposite end. The latter approach has the advantage of being able to accommodate changes in the cable plant, the receiver, and the laser-to-fiber coupling. It has the disadvantage of requiring the addition of control circuitry at the receiver and a link back to the transmitter. If the feedback link is already present, as it would be for a fiber-to-the-curb application, such as disclosed in U.S. patent application Ser. No.07/739,203, entitled "Fiber Optic Link", filed Jul. 30, 1991, now abandoned, capacity may be used for feedback information. However, the possibility that feedback information may not arrive back to the laser in a timely fashion, causing instability in the laser performance, must also be considered.
Localized feedback through the back facet monitor is commonly used to regulate the bias current of the laser. Slope efficiency variations, which are as high as 6 dB, are often ignored. In some cases, through the generation and addition of a fixed level `pilot` carrier to the signal, modulation current is also regulated through the back facet monitor diode. However, circuitry must be added to generate the pilot carrier and a multiplexer with the signal. Additionally, link bandwidth is taken up by the pilot.